1. Field of the Invention
The present invention relates to a switching device such as a microswitch having a snap-action function.
2. Description of the Related Art
As shown in FIG. 24, an existing switching device of this type comprises a case 60, a common terminal 61 attached to the case 60, and fixed terminals 62, 63 attached to the case 60. One end of a receiving member 64 is pivotably engaged with an engagement section 61a for the purpose of supporting the common terminal 61. A movable leaf retaining section 64a of the receiving member 64 pivotably supports a movable leaf 65. The common terminal 61 receives one end of a movable spring 66, and the other end of the movable spring 66 is engaged with a spring engagement section 65a of the movable leaf 65. The inner end of an actuation plunger 67 provided for the case 60 is brought into contact with a free end of the receiving member 64, and a cover (not shown) is fitted to the case 60.
Such a switching device has a snap-action function, so that a switching point is immediately reversed when the actuation plunger 67 is actuated to an operation position. The snap-action mechanism includes the receiving member 64 that is engaged at one end with the engagement section 61a of the common terminal 61, as previously described; the movable leaf 65 pivotably engaged with the movable leaf retaining section 64a of the receiving member 64; the movable spring 66 that is engaged at one end with the common terminal 61 and engaged at the other end with the spring engagement section 65a of the movable leaf 65; and the actuation plunger 67 whose inner end is brought into contact with a free end of the receiving member 64. As a result of the depression of the actuation plunger 67, a point of joint S between the movable leaf 65 and the receiving member 64 is moved beyond a point of joint R between the movable spring 66 and the common terminal 61.
In the foregoing existing switching device, in order to retain the snap-action characteristics of the switching device, the one end of the receiving member 64 is engaged with the engagement section 61a of the common terminal 61, and the movable leaf retaining section 64a of the receiving member 64 is pivotably engaged with and retains the movable leaf 65. The movable spring 66 is engaged at one end with the common terminal 61 and is engaged at the other end with the spring engagement section 65a of the movable leaf 65. In this way, the components are assembled into one device only through engagement.
When an electrical current flows through the engagement section between the components, heat develops in the engagement section as a result of constriction resistance. For this reason, the primary current-carrying section constructed by the common terminal 61, the receiving member 64, and the movable leaf 65 must be made of material having superior electrical conductivity, or the conductivity of the primary current-carrying section must be increased by plating, thereby adding to the cost. Further, if the engagement sections are fixed, the problem associated with conductivity will be solved. However, the reactive force caused by snap action reduces the lifetime of the components.